Many communications systems include apparatus which monitors the call processing equipment to detect service irregularities. Thus, if faulty call processing equipment is detected, the equipment can be taken out of service, and new equipment substituted. Problems exist, however, when the units of equipment encounter faults during different stages of call processing.
For example, if a fault occurs before the unit of equipment is engaged on a call, the defective unit can be removed from service and a spare unit selected. Under these circumstances the customer using the system would be unaware of the equipment failure.
If, on the other hand, a unit of equipment fails during call processing, the call will be disrupted. In some instances the customer is unaware of the failure, except that the call is not completed while in other instances the call may time-out and a tone be returned to the customer.
While the use of tones for informing the customer of equipment trouble is wholly suitable for its intended purposes, these tones are also used to inform the customer of other conditions, such as all circuits busy and thus might confuse the customer as how to proceed when trouble is encountered.